1. Field of the Invention
This invention relates to a scanning optical system having an imaging optical system including a lens element formed of a plastic material.
2. Description of the Prior Art
If plastic materials represented by acryl are used, optical parts can be processed at very much lower cost by molding or the like using a metal mold than can optical parts made by the method of polishing glass. Also, optical parts of non-spherical shape which are difficult to process by glass polishing can be mass-produced relatively easily.
However, it is well known that plastic optical members formed of acryl or polystyrene, on the other hand, suffer from a serious problem that as compared with glass, they exhibit great fluctuation in their optical characteristics as a result of variations in environmental conditions such as temperature and humidity.
Description will hereinafter be made of the practical problems which will be encountered when a lens formed of such plastic material having a moisture absorbing property is used in the interior of a scanning optical system.
FIG. 1 of the accompanying drawings shows the arrangement of a scanning optical system. The light beam emitted from a light source portion 1 is deflected by a deflector 2 in a plane orthogonal to the rotary shaft of the deflector 2. The deflected light beam is formed as an imaged spot on a medium 4 to be scanned by a lens group 3, thereby effecting flying spot scanning.
In the above-described optical system, the lens group 3 not only images a spot on the medium to be scanned, but also has the important function of imparting a strain characteristic peculiar to the scanning optical system.
What should be noted is the light beam passage area in each lens within the lens group 3 and, because the light beam deflected by the deflector passes through it, it is necessarily longer in the deflection direction x as shown by the hatching in FIG. 2 and shorter in the direction y orthogonal to the deflection direction. Accordingly, the shape of each lens may be made such that in order to cover the light beam passage area, it is longer in the deflection direction x and shorter in the direction y orthogonal to the deflection direction, as shown in FIG. 2, whereby the optical system becomes compact and moreover, manufacture of the lenses becomes easy.
If the material of the lens having a cross-sectional shape perpendicular to the optical axis as shown in FIG. 2 is plastic, molding can be easily accomplished by compression, injection or casting.
In contrast, FIG. 3 shows a case where the cross-sectional shape of a lens perpendicular to the optical axis is circular, and it is obvious in this figure that, as compared with FIG. 2, there are many areas of the lens other than the light beam passage area indicated by hatching. If the less wasteful shape of FIG. 2, instead of that of the shape of FIG. 3, is adopted, not only the construction of the molding apparatus can be simplified but also the molding time can be reduced.
In the scanning optical system of U.S. Pat. No. 3,750,189, there is disclosed a lens having a shape longer in the deflection direction of light beam. If a plastic material such as acryl having a moisture absorbing property is used for such a lens, there will occur a problem that moisture absorption affects the imaging performance of the lens.
Forming a lens into a shape longer in the deflection direction is very effective in molding as noted above, but as compared with a lens having the cross-sectional shape as shown in FIG. 3, it suffers from great influence of moisture absorption in the direction y orthogonal to the deflection plane.